1. Field of the Invention
This invention is directed to a hydrogen-resistant optical fiber cable suitable for underwater use, and a method for making underwater hydrogen-resistant optical fiber cable using terrestrial optical fiber cable.
2. Description of the Related Art
Consumers have widely varying requirements in selecting underwater optical fiber cable. The consumer's intended application may require a certain minimum fiber count, the inclusion of certain materials to provide protection from the environment of use, the presence of conductors to supply power to optical repeaters, certain tensile strengths and/or numbers of layers of armor wires to prevent the cable from being damaged, etc. At present, consumers can select from only a few commercially available alternatives of underwater optical fiber cable. Consequently, consumers are often compelled to pay for expensive features of an underwater optical fiber cable that are not needed for his particular application. To eliminate unnecessary, expensive features, a consumer can design a customized underwater optical fiber cable specifically fitting the application requirements. However, such a design process often involves extensive research and development with an associated expense that tends to negate any savings that might accrue to the consumer by designing, as opposed to buying a preexisting underwater optical fiber cable. Therefore, there is a need among consumers in the optical fiber cable industry for a wider range of choice of underwater optical fiber cables for such features as cable type and fiber count, and preferably, as wide a range of choice as is available to consumers of terrestrial optical fiber cable.
Even those few underwater optical fiber cables that are commercially available to consumers suffer from drawbacks in design or expense in relation to the problem of hydrogen-generation in underwater optical fiber cables. Optical fiber cables often include galvanized armor wires to reinforce the cable, that have been found to generate hydrogen gas when contacted by water (H.sub.2 O). This hydrogen can pass through the cable into its core and permeate the optical fibers. Hydrogen-permeated optical fibers lose their transmissivity and greatly attenuate light passing therethrough, thus requiring much greater power for optical data transmission, reduced spacing between repeaters, or possibly even rendering the optical fibers unusable.
One approach to solving this problem has focused on reducing or preventing the generation of hydrogen in the materials composing the optical fiber cable. This can be done by using an optical fiber cable oversheath with stainless steel armor wires that are less prone to generating hydrogen than are galvanized armor wires. Nonetheless, the stainless steel armor wires can generate hydrogen under certain conditions if contacted by water. Accordingly, stainless steel armor wires have been coated with plastic to prevent the contact of water with the metal (see, e.g., U.S. Pat. No. 4,974,926 of John J. Blee, et al.). Although this approach has been proven effective in preventing hydrogen-generation, it is not without extra cost. Stainless steel armor wires with plastic coatings significantly increase the cost of an optical fiber cable.
Another approach to solving the hydrogen-generation problem while retaining the flexibility of the many types of low cost, high fiber count terrestrial cable is to convert a standard terrestrial cable to a robust underwater cable by using a hydrogen barrier such as a welded, corrugated copper tube to enclose hermetically the preexisting terrestrial cable. This approach, used in the present invention, permits the use of a relatively inexpensive galvanized armor wire in the optical fiber cable oversheath because the copper tube provides a hydrogen barrier to protect the terrestrial cables from hydrogen contamination. With the use of galvanized armor wire in the oversheath, significant cost reduction is obtained, and further, any hydrogen generated by the galvanized armor wire is prevented from permeating the optical fibers.
In addition to the flexibility of providing a myriad of fiber core configurations immediately by merely choosing the appropriate existing terrestrial cable, the terrestrial cable in the core provides a "stand-alone" cable that can continue and/or form a standard connection with like cable on the shore ends.
Besides the functional conformance advantages of optimizing the core cable for the task, the core cable can also be chosen such that they have no metal and/or wire reinforcement inside of the hydrogen barrier. In the event of a leak or rupture in the hydrogen barrier allowing water to enter, this metal or wire could generate hydrogen in close proximity to optical fibers. This hydrogen is likely to contaminate the optical fibers, reducing their performance or rendering them unusable. It would be desirable to eliminate this deficiency of most presently-known optical fiber cables that use a hydrogen barrier.